1. Field of the Invention
This invention relates to a control system for a legged mobile robot, more particularly to such a system for enabling a biped walking robot to walk stably even over terrain with unexpected irregularities.
2. Description of the Prior Art
Wheeled, crawler, legged and other types of mobile robots have been proposed. Among the numerous reports published regarding control technologies for the legged mobile robot are included some relating to the one-legged robot (Raibert, M. H. , Brown, Jr. H. B. , "Experiments in Balance with a 2D One-Legged Hopping Machine", ASME, J of DSMC, vol. 106, pp. 75-81 (1984)), the two-legged robot (J of the Robotic Society of Japan, vol. 1, no. 3, pp. 167-203 (1983)), the four-legged robot (J of the Robotic Society of Japan, vol. 9, no. 5, pp. 638-643 (1991)), and the six-legged robot (Fischeti, M. A., "Robots Do the Dirty Work," IEEE, Spectrum, vol. 22, no. 4, pp. 65-72 (1985) and Shin-Min Song, Kenneth J. Waldron, "Machines That Walk; The Adaptive Suspension Vehicle", The MIT Press Cambridge, Mass., London, England). Other reports have been published regarding techniques for real time generation of a dynamically stable motion (walking) pattern for a robot with relatively few degrees of freedom (Shimoyama, "Dynamic Walking of Stilt-type Biped Walking Robot", Collected Papers of The Japan Society of Mechanical Engineers, book C, vol. 48, no. 433, pp. 1445-1454 (1982) and "Legged Robots on Rough Terrain; Experiments in Adjusting Step Length", by Jessica Hodgins, IEEE, (1988), while techniques for offline generation of a stable motion (walking) pattern for a robot with relatively many degrees of freedom (Japanese Laid-open Patent Publication Nos. 62(1987)-97006 and 63(1988)-150176).
Among them, the anthropoid biped robot comprises a main body unit to which two leg links are connected. The robot walks by repeatedly swinging first one and then the other leg link forward and setting the foot on the ground. This locomotion is ordinarily achieved by designing a locomotion pattern in advanced based on the assumption that the robot will walk over a smooth terrain, converting the pattern to joint angle trajectories, and controlling the joint angles so as to follow the trajectories without delay. The same applies regarding known stairs and inclines.
This prior art technology leads to problems when the terrain to be navigated includes unexpected (unknown) irregularities (bumps and depressions). While the foot angle of the robot's supporting leg is always constant when walking on a known terrain, this is not true on irregular terrain and, as a result, it is impossible for the robot to walk stably in accordance with the locomotion pattern designed with a specific known terrain in mind. In extreme cases, the robot may tip over. One technique for enabling stable topple-free walking on a terrain with unknown irregularities is to design a walking pattern that defines the robot attitude from the foot upward and then control the actual attitude to follow the designed attitude. This technique does not control the robot joint angles but controls the absolute angles of the links from the foot of the supporting leg upward, which is to say that it controls the robot's attitude with respect to the direction of gravity. This is possible because no matter what the angle of the supporting foot is, stable locomotion is ensured insofar as the target attitude (i.e. an attitude not susceptible to tip-over) is established above the foot. The assignee earlier proposed a system based on this concept in Japanese Patent Application No. 2(1990)-259,839 which was also filed in the United States under the number of 766,629 on Sep. 26, 1991.
This earlier technology proposed by the assignee approximates the robot's motion using a linear model. Therefore, for realizing smooth walking in accordance with a walking pattern in which the joint at the part of the supporting leg corresponding to the human knee is required to make very large movements or with a walking pattern involving large strides which cause major deviation from linearity, it is necessary to compensate for nonlinearity, as is often seen in the control of a manipulator of a stationary robot. This, however, requires a high-speed computer to be installed in the robot's control unit, which is undesirable for a mobile robot because of the large size and heavy weight of such a computer.